Interim Estimates of 2023–24 Seasonal Influenza Vaccine Effectiveness — United States

In the United States, annual influenza vaccination is recommended for all persons aged ≥6 months. Using data from four vaccine effectiveness (VE) networks during the 2023-24 influenza season, interim influenza VE was estimated among patients aged ≥6 months with acute respiratory illness-associated medical encounters using a test-negative case-control study design. Among children and adolescents aged 6 months-17 years, VE against influenza-associated outpatient visits ranged from 59% to 67% and against influenza-associated hospitalization ranged from 52% to 61%. Among adults aged ≥18 years, VE against influenza-associated outpatient visits ranged from 33% to 49% and against hospitalization from 41% to 44%. VE against influenza A ranged from 46% to 59% for children and adolescents and from 27% to 46% for adults across settings. VE against influenza B ranged from 64% to 89% for pediatric patients in outpatient settings and from 60% to 78% for all adults across settings. These findings demonstrate that the 2023-24 seasonal influenza vaccine is effective at reducing the risk for medically attended influenza virus infection. CDC recommends that all persons aged ≥6 months who have not yet been vaccinated this season get vaccinated while influenza circulates locally.


Introduction
CDC's Advisory Committee on Immunization Practices recommends annual influenza vaccination for all persons aged ≥6 months (1).During previous influenza seasons, influenza vaccination prevented hundreds of thousands of outpatient medical visits, tens of thousands of hospitalizations, and thousands of deaths from influenza.*During the current influenza season, most influenza viruses detected were influenza A(H1N1)pdm09 viruses with cocirculation of influenza B/Victoria and influenza A(H3N2).† Because circulating seasonal influenza viruses change continuously, influenza vaccines are reviewed biannually and updated as needed.CDC has monitored the effectiveness of annual influenza vaccines against circulating influenza strains since 2004.§ This report provides interim estimates of 2023-24 seasonal influenza vaccine effectiveness (VE) against laboratory-confirmed influenza for children, adolescents, and adults in the outpatient and inpatient settings from active and passive surveillance systems in 22 U.S. states.
IVY enrolled adult patients admitted to the hospital (Box).NVSN enrolled pediatric patients who received outpatient care** (outpatient clinics, urgent care, and emergency departments), and those admitted to the hospital.US Flu VE enrolled pediatric and adult patients who received outpatient care (outpatient clinics, urgent care, and emergency departments).VISION included pediatric and adult patients who received outpatient care (urgent care and emergency departments), and those admitted to the hospital.

Data Analysis
Influenza VE was estimated based on a test-negative casecontrol design using multivariable logistic regression as (1 -adjusted odds ratio) × 100%.Case-patients were those with ARI who received a positive † † influenza molecular assay test result.Control patients were those with ARI who received a negative influenza molecular assay test result.Patients were considered vaccinated § § if they had received ≥1 dose of 2023-24 influenza vaccine ≥14 days before an index date.¶ ¶ Patients were excluded*** if they were vaccinated within 13 days of the index date or received a positive SARS-CoV-2 test result (6).VE estimates were calculated for influenza A subtypes A(H1N1)pdm09 and A(H3N2) when possible.If more than one network had a VE estimate for the same age group, influenza type, and setting, VE was reported in the text as a range, from lowest VE point estimate to highest, without CIs.
Logistic regression models were adjusted for geographic region, age, calendar time of illness, † † † and other prespecified confounders.§ § § SAS software (version 9.4; SAS Institute) and R (version 4.3; R Foundation) were used to conduct the analyses.IVY, NVSN, and US Flu VE activities were reviewed by CDC, deemed not research, and were conducted consistent with applicable federal law and CDC policy.¶ ¶ ¶ VISION activities were reviewed and approved by the Kaiser Permanente Northern California, Kaiser Permanente Southern California, and Westat institutional review boards.****

Vaccination Status Among Control Patients
During the 2023-24 influenza season, the proportion of patients with medically attended ARI who had received † † All influenza case-patients received a positive reverse transcription-polymerase chain reaction test result from a clinical or surveillance respiratory laboratory specimen for IVY, NVSN, and US Flu VE.For VISION, influenza casepatients received a positive molecular assay result from a clinical respiratory laboratory specimen.§ § Vaccination status was self-or parent-or guardian-reported or abstracted from medical records, immunization information systems, or claims data.influenza vaccine varied by VE network, patient age, and setting.Among pediatric patients, the proportion of vaccinated control patients within the VE networks ranged from 25% to 31% in outpatient settings and from 32% to 41% in the inpatient setting.Among adult control patients aged 18-64 years, 28% to 37% in outpatient and 30% to 34% in inpatient settings were vaccinated; among control patients aged ≥65 years, 62%-68% in outpatient and 48%-60% in inpatient settings were vaccinated.

Pediatric VE
VE against any influenza-associated ARI for children and adolescents aged 6 months-17 years ranged from 59% to 67% in outpatient settings and from 52% to 61% against any influenza-associated hospitalization (Table 1).VE against influenza A ranged from 46% to 59% in outpatient settings and from 46% to 56% against influenza-associated hospitalization.VE against influenza A(H1N1)pdm09 ranged from 54% to 61% in outpatient settings and against influenza-associated hospitalization was 60%.VE against A(H3N2) was 55% in outpatient settings.VE against influenza B ranged from 64% to 89% in outpatient settings.

Adult VE
VE against any influenza-associated ARI for all adults aged ≥18 years ranged from 33% to 49% in outpatient settings and from 41% to 44% against any influenza-associated hospitalization (Table 2).VE against influenza A ranged from 27% to 46% in outpatient settings and from 40% to 42% against influenza-associated hospitalization.VE against influenza A(H1N1)pdm09 was 25% in outpatient settings and 50% against influenza-associated hospitalization.VE against influenza A(H3N2) was 54% in outpatient settings.VE against influenza B was 78% in two networks in outpatient settings and was 60% against influenza-associated hospitalization.
VE against any influenza-associated ARI for adults aged 18-64 years ranged from 25% to 52% in outpatient settings and from 40% to 49% against any influenza-associated hospitalization.VE against any influenza A ranged from 13% to 49% in outpatient settings and from 38% to 42% against influenza-associated hospitalization.VE against influenza B ranged from 75% to 79% in outpatient settings and was 50% against influenza-associated hospitalization.
VE against any influenza-associated ARI for adults aged ≥65 years ranged from 41% to 51% in outpatient settings and in two networks was 42% against any influenza-associated hospitalization.VE against influenza A ranged from 40% to 52% in outpatient settings and from 42% to 47% against influenza-associated hospitalization.VE against influenza B was 69% in outpatient settings.

Discussion
These interim estimates indicate that receipt of 2023-24 influenza vaccination reduced the risk for medically attended influenza-associated outpatient visits and hospitalization among children and adolescents and among adults, including those aged ≥65 years, consistent with results from previous years.† † † † Influenza vaccination was effective against both influenza A (mostly subtype A(H1N1)pdm09) and B (lineage Victoria) viruses that have circulated so far this season, consistent with recent findings from Canada and Europe (7,8).VE estimates among adults ≥65 years, a group at increased risk for severe illness (1), were similar to those among adults aged 18-64 years.These findings support continuing efforts to increase influenza vaccination coverage to prevent influenza † † † † https://www.cdc.gov/flu/vaccines-work/effectiveness-studies.htmInfluenza vaccination coverage in the United States has been lower this season than in the previous season and also lower than coverage before the COVID-19 pandemic.§ § § § In the current analyses, fewer than one half of test-negative control patients had received influenza vaccine in all VE networks and among enrollees of most age groups.The public health benefit of annual influenza vaccination depends on both vaccine effectiveness and vaccination coverage.Increased vaccination coverage will maximize prevention of influenza-associated illness, reducing both outpatient visits and hospitalization (9,10).§ § § § https://emergency.cdc.gov/han/2023/han00503.aspThis is the first time interim pediatric and adult influenza VE estimates from four major networks have been presented together.Whereas previous interim VE estimates were for outpatient settings only, these analyses include estimates of VE among children and adolescents and among adults across a spectrum of illness severity.These findings are further strengthened by the geographic diversity of the networks, representing patients in 22 U.S. states.

Limitations
The findings in this report are subject to at least four limitations.First, small sample sizes prevented estimation of VE for some age groups and settings.For example, an estimate of VE against influenza A(H3N2) was only possible in outpatient settings.Second, although models were adjusted for potential confounders, the potential for unmeasured confounding remained, such as underlying medical conditions or prior vaccination status.Third, there might be misclassification of vaccination status for networks that used self-reported vaccination data or if vaccine was administered outside of the medical system.Finally, in these analyses, patients who received ≥1 dose of 2023-24 influenza vaccine were considered vaccinated.However, to be considered fully vaccinated for the season, children aged 6 months-8 years are recommended to receive 2 influenza vaccine doses if they have not been previously vaccinated (1).Thus, some children classified as vaccinated might have only been partially vaccinated.

Implications for Public Health Practice
Influenza vaccination remains the best way to prevent influenza.These findings provide further evidence of the importance of influenza vaccination in reducing medically attended influenza illness in outpatient and inpatient settings among all age groups.Last year alone, CDC estimates that influenza vaccination prevented about 6 million illnesses, 65,000 hospitalizations, and 3,700 deaths.¶ ¶ ¶ ¶ These findings support the recommendation for all persons aged ≥6 months to be vaccinated against influenza (1).

Inpatient versus outpatient settings: Inpatient, urgent care clinics, and EDs • Type of surveillance: Passive • Medical centers included (state): HealthPartners
Abbreviations: ARI = acute respiratory illness; Ctr.= Center; ED = emergency department; ICD-10 = International Classification of Diseases, Tenth Revision; IVY = Investigating Respiratory Viruses in the Acutely Ill Network; Med.= Medical; NVSN = New Vaccine Surveillance Network; Univ.= University; US Flu VE = United States Influenza Vaccine Effectiveness Network; VISION = Virtual SARS-CoV-2, Influenza, and Other respiratory viruses Network.
¶ ¶ Index date for IVY, NVSN, and US Flu VE was date of ARI onset.Index date for VISION was the earlier of outpatient visit, hospital admission date, or influenza clinical testing date.

TABLE 1 . Number and percentage of children and adolescents aged 6 months-17 years receiving seasonal influenza vaccine, number and percentage with a positive or negative influenza test result, and vaccine effectiveness,* by influenza type † and subtype § -three networks, United States, 2023-24 influenza season
All networks adjusted for geographic region, age, and calendar time.US Flu VE and VISION adjusted for sex and race and ethnicity.US Flu VE also adjusted for time since illness onset and self-reported health status.VE estimates with fewer than 50 cases or from models that did not converge are not presented and are indicated with a dash.
¶ ** Patients enrolled as outpatients in NVSN might have progressed to a more acute level of care, and those data might not be reflected in this analysis.† † For NVSN and US Flu VE, outpatient setting is defined as outpatient clinics, urgent care, and emergency departments; for VISION, an outpatient setting is defined as urgent care and emergency departments.

TABLE 2 . (Continued) Number and percentage of adults aged ≥18 years receiving seasonal influenza vaccine, number and percentage with a positive or negative influenza test result, and vaccine effectiveness* by influenza type † and subtype § -three networks, United States, 2023-24 influenza season
All networks adjusted for geographic region, age, and calendar time.IVY, US Flu VE, and VISION, adjusted for sex and race and ethnicity.US Flu VE also adjusted for time since illness onset and self-reported health status.VE estimates with fewer than 50 cases or from models that did not converge are not presented and are indicated with a dash.** For US Flu VE, outpatient setting is defined as outpatient clinics, urgent care, and emergency departments; for VISION, an outpatient setting is defined as urgent care and emergency departments. ¶ Adam S. Lauring, University of Michigan; Julie Arndorfer, Intermountain Medical Center; Daniel Bride, Intermountain Medical Center; Ithan D. Peltan, Intermountain Medical Center; Nicholas M. Mohr, University of Iowa; David N. Hager, Johns Hopkins University; Matthew Prekker, Hennepin County Medical Center; Amira Mohamed, Montefiore Medical Center; Nicholas Johnson, University of Washington; Jay Steingrub, Baystate Medical Center; Akram Khan, Oregon Health and Science University; Laurence W. Busse, Emory University; Abhijit Duggal, Cleveland Clinic; Jennifer G. Wilson, ¶ ¶ ¶ ¶ https://www.cdc.gov/flu/about/burden-prevented/2022-2023.htm